a. Field of the Invention
The present invention relates generally to an ablation catheter and method for using ablation catheters. In particular, the present invention relates to methods and devices for measuring tissue cooling and for estimating lesion characteristics as the lesion is being formed.
b. Background Art
Catheters are used for an ever growing number of medical procedures. To name just a few examples, catheters are used for diagnostic, therapeutic, and ablation procedures. Typically, the user manually manipulates the catheter through the patient's vasculature to the intended site, such as a site within the patient's heart. The catheter typically carries one or more electrodes or other diagnostic or therapeutic devices, which may be used for ablation, diagnosis, cardiac mapping, or the like.
It is well known to utilize catheters for ablation treatments. For example, catheters may be used to accomplish ablation by transmission of ablative energy to a desired target area through an electrode assembly to ablate tissue at the target site. Ablative energy may generate significant heat, which if not controlled, can result in excessive tissue damage, such as steam pop, tissue charring, and the like. Accordingly, a need exists to control the delivery of ablation energy and to prevent undesired tissue damage.
Surgical devices and techniques utilizing electrodes to transfer therapeutic energy to tissue are well known. Electrosurgery allows for the incision, cauterization, fulguration, and desiccation of tissue through the application of high-power, radio frequency (RF) energy to tissue through an electrode. Ablation techniques, whereby the target tissue is necrotized through coagulation, are also performed using surgical devices with electrodes to transfer RF energy to tissue. Many benefits may be gained by forming lesions in tissue—for example, control of cardiac arrhythmia or tachycardia, removal of skin diseases, or the treatment of varicose veins—if the depth and location of the lesions being formed can be controlled. In particular, it can be desirable to elevate tissue temperature to around 50-55° C. until lesions are formed via thermal necrosis, which changes the electrical properties of the tissue. For example, when sufficiently deep lesions are formed at specific locations in cardiac tissue via coagulation necrosis, undesirable ventricular tachycardia may be lessened or eliminated. “Sufficiently deep” lesions means transmural lesions in some cardiac applications.
Several difficulties may be encountered, however, when attempting to form adequately-deep lesions at specific locations using some existing surgical ablation electrodes. For example, when forming lesions with ablation energy, high temperature gradients are often encountered in the vicinity of the electrode. At the edges of some existing electrodes are regions of very high current density leading to large temperature gradients and hot spots. These “edge effects” may result in the formation of undesirable coagulum and charring of the surface tissue. For example, undesirable coagulum may begin to form when blood reaches around 80° C. for an appreciable length of time, and undesirable tissue charring and desiccation may be seen when tissue reaches around 100° C. for an appreciable length of time. There two types of undesirable coagulum: coagulum that adheres to and damages the medical device; and coagulum blood clots or curds that may enter a patient's bloodstream, possibly resulting in other health problems for the patient. Charring of the surface tissue may also have deleterious effects on a patient.
As the temperature of the electrode is increased, the contact time required to form an adequately-deep lesion decreases, but the likelihood of charring surface tissue and forming undesirable coagulum increases. As the temperature of the electrode is decreased, the contact time required to form an adequately-deep lesion increases, but the likelihood of charring surface tissue and forming undesirable coagulum decreases. Moreover, the use of fluids in the ablation process further complicate the process especially when the application of fluid results in tissue cooling. It is, therefore, a balancing act trying to ensure that tissue temperatures are adequately high for long enough to create deep lesions, while still preventing or minimizing coagulum formation and/or charring of the surface tissue.
Generally, a need exists to control the delivery of ablation energy and to provide greater feedback on tissue temperature, as well as size and other lesion forming characteristics during the ablation process.
The information included in this background section of the specification, including any references cited herein and any description or discussion thereof, is included for technical reference purposes only and is not to be regarded subject matter by which the scope of the invention is to be bound.